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REVETT MINERALS program, we ended last year with little more than seven years." The Troy mill has to process around 100 tons of ore to produce 1 ton of concentrates. "A fair amount of reserves remain in the C, A and I Beds," said Larry Erickson, exploration and technical serv- ices manager, Revett Minerals. "Revett purchased some claims from Kennecott when it purchased the property. We now have more than 500 unpatented claims. We have a property to the south of Ross Creek, which has a historic resource of about 11 million tons." One of two 15.5-ft ball mills at the Troy mill, which processes 4,000 tpd. and Tier 4 engines as we can," Miller said. "Unfortunately, Tier 4 engines have not yet been developed for the higher horsepower equipment in the 600- to 650-hp range. Those units are still pow- ered by Tier 2 diesels." In 2008, the U.S. Mine Safety and Health Administration decreased the threshold for DPM to 160 micro- grams/m3. Underground metal mines, such as Troy, had several options. They could increase the amount of air moving through the mine, add exhaust purifiers, upgrade the diesel engines, use bio- diesel, or any combination thereof. "We decided to use all of the tools at our dis- posal to consistently meet the new DPM standard," Miller said. "We recently com- pleted a lengthy process of implementing the use of biodiesel. We started with B5 canola oil. Today, we are burning B75, where 75% of the fuel is Canola." An exhausting fan at a portal in the south ore body draws 480,000 cfm of clean air through the mine from portals in the north ore body. After 12 years on care and mainte- nance, Miller said they encountered few surprises during the restart process. "The mill restarted easily," Miller said. "A por- tion of the mine was flooded as one would expect. The biggest challenge was finding the skilled workforce. This is a 66 E&MJ; • JUNE 2012 fairly remote area and getting the people that had been here in the past to return was difficult. Those families were now established in other areas." Attracting new miners was difficult too. At the time, the mining business was picking up in Nevada. "In addition to being isolated, the region is so moun- tainous, people sometimes feel hemmed in here," Miller said. "Once we had new miners in place, the training takes time. It takes about four or five years to train a skilled miner. Truck driving is a little easier, but the drills and loaders required more training and skill sets. We were able to find some operators from the timber industry." The mine currently employs 195 peo- ple. "We have a fairly aggressive explo- ration plan and we have the development work, so we have a few more people work- ing with us than we normally would and we're looking to hire a few more for the next couple of years," Miller said. An Aggressive Exploration Program The current life for the Troy mine is 7.5 years and 10.5 million tons. "When we restarted the mine in 2004, we had a little more than a three-year mine life," Miller said. "Due to increased metal prices and the expanded exploration Part of current exploration program, Erickson explained, is to continue to define a continuation of the mineraliza- tion defined in the I Bed under the Troy mine to the I Bed in the new JF Property. "We performed some geophys- ical surveys as well as re-interpreted some previous geophysical work, and revisited the old core samples," Erickson said. "We have done quite a bit of new drilling particularly from the south end of the mine. Then we plan to drill from private property onto the JF Property and upgrade the statistics and add more resources to our current 10 million tons." This could significantly extend the life for the Troy mine. "We have a lot of mineralization in the I Bed and we believe there might be a core of mineable higher grade mineral- ization," Miller said. "It is not only in the southern direction, but also trailing off to the north." The Troy Mill Improves with Age The ore body consists of a finely dissem- inated copper. A primary crusher under- ground (an Allis Chalmers jaw crusher) reduces ore to 6-in. A conveyor transports ore to a secondary crusher on the surface, which reduces the rock to minus 5/8-in. The secondary crushers, which consists of three 7-ft Symons (one standard and two short head cone crushers), feed a fine ore bin. The rock coming into the mill is then ground to a fine consistency (P80 = 100 microns) to liberate the minerals. The original nameplate capacity for the Troy mill was 8,500 tpd. The mill uses two fine grinding circuits, which consist of parallel trains with a 2,500- hp, 15.5-ft dia., 18-ft long Kennedy Van Saun ball mill. "We typically run around 4,000 tpd," said Steve Lloyd, mill man- ager, Revett Minerals. "So, we have the www.e-mj.com